In general, maintaining the stable yaw orientation of a helicopter in hover or low speed flight can be a difficult business for the pilot. To counterbalance the constantly changing torques on the helicopter fuselage produced by the main rotor blades and atmospheric conditions such as lateral wind gusts, helicopter pilots must continually manipulate the yaw controls of their aircraft.
Conventionally, the pilot of a full size helicopter controls the tail rotors by manipulating foot pedals located within the cockpit. Cables, push-pull rods, and bellcranks connect the pedals to the collective pitch controls of the tail rotor blades. As the pilot adjusts the pedal position, the change in angle of attack (pitch) and associated thrust force of the rotating tail rotor blades results in a yaw moment about the center of gravity of the helicopter. This moment is directed to maneuver the helicopter, or to oppose any destabilizing yaw moment sensed by the pilot.
Tail rotors of radio-controlled model helicopters operate in a manner identical to full size helicopters. The pilot manipulates a hand-held radio transmitter which in turn sends commands to electro-mechanical servo actuators located within the flying model. Push-pull rods and bellcranks connect the servos to the collective pitch controls of the tail rotor blades. Yaw instability can make a model helicopter particularly difficult for the pilot to control. This is because the pilot manipulates controls affixed to the radio transmitter, not to the model, so flight controls for yaw, roll and fore-aft cyclic are effectively reversed when the nose of the model becomes oriented toward the pilot.
To control yaw instability, both full-size and model helicopters are frequently equipped with stabilizer systems. Gyro-stabilizer systems can be broadly classified as either mechanical or electro-mechanical. Mechanical systems generally rely on precessional (angular) displacement of a relatively large gyroscopic arm or flywheel mechanism to alter the pitch of the tail rotor blades in opposition to any yaw displacement of the helicopter. Electro-mechanical systems sense the precessional displacement of a relatively small flywheel mechanism, and control the tail rotor blades through electronic amplification and electro-mechanical and/or hydraulic servo actuators. Modern model helicopters frequently carry electro-mechanical gyro stabilizer systems which are electronically mixed into the tail rotor servo control circuit. These gyro systems are relatively expensive and heavy, and draw power from the airborne radio receiver system batteries. An example of an electro-mechanical system designed for full-size helicopters is described in U.S. Pat. No. 3,528,633.
Some yaw stabilizer systems, especially more sophisticated electro-mechanical systems, disengage whenever the pilot maneuvers the aircraft. Other systems, most notably mechanical systems, act to suppress all yaw motion of the helicopter including that desired by the pilot. With these mechanical systems the pilot must forcibly override the gyroscopic mechanism in order to control the tail rotor for trimming and normal flight. Since gyroscopic mechanisms tend to resist displacement, the pilot will feel resistance to control inputs. This resistance will typically persist as long as the rate of yaw is not zero. Generally, these systems tend to increase stability at the expense of controllability.
One such mechanical device is shown in U.S. Pat. No. 3,004,736. The mechanism includes a gyroscopic mass in the form of weighted arms extending radially from and fixed via a gimbal to a rotating splined shaft which in turn is connected to the tail rotor pitch control mechanism. Precession (tilt) of the rotating arms about an axis perpendicular to and offset from the axis of rotation displaces the splined shaft axially thereby altering the pitch of the tail rotor blades. Override springs are provided on the tail rotor control cables to accommodate axial movement of the splined shaft. Pilot control inputs must forcibly change or override the gyroscopic mechanism in order to maneuver the aircraft. A related mechanical gyro stabilizer mechanism is detailed on page 41 of the March 1973 issue of American Aircraft Modeler magazine (originally located at 733 15th Street N.W., Washington, D.C. 20005). In this mechanism yaw moment applied to a gyroscopic ring causes the ring to precess (tilt) off from the vertical about an offset axis. Displacement of the ring moves a slider on the tail rotor shaft and changes the pitch of the tail rotor blades to counter the yaw moment. This mechanism also suppresses pilot inputs, and requires override springs, ball bearings, pivot linkages, a gimbal mechanism and specially designed tail boom structure.
Another mechanical gyro stabilizer system is described in U.S. Pat. No. 4,759,514. This mechanism relies on gyroscopic precession of the entire tail rotor assembly about an offset axis to displace a slider connected to the tail rotor blades. This system differs from the aforementioned mechanical systems in that stabilizer control inputs are mechanically mixed with, rather than overridden by, pilot control inputs. Obvious drawbacks to this system include the complexity of the tail rotor mounting structure, and the required universal joint incorporated into the tail rotor drive shaft.
Other references to helicopter tail rotor control and stabilizer systems include U.S. Pat. No. 3,211,235 which describes a basic tail rotor control system; U.S. Pat. No. 3,532,302 which describes the use on a military helicopter of a spring loaded actuator to control tail rotor pitch adequately for a return flight to base if the primary control linkage system fails; and U.S. Pat. No. 4,272,041 which describes a non-gyroscopic technique for reducing transient yaw instability in model helicopters using a complex system of gears, levers and push-pull rods to sense and correct for torque changes.
These and similar yaw stabilizer systems currently available suffer from one or more disadvantages. Mechanical designs rely on expensive multiple ball bearings, complicated gimbal mechanisms, specially designed sliding shafting, and specially designed tail boom structure and pivoting mechanisms. Many require some sort of override springs on the pilot yaw control cables which must be carefully adjusted or "tuned". Stiff springs dampen gyro effectiveness while overly elastic springs dangerously decrease pilot control. Electro-mechanical systems are heavy and expensive in model applications, and require servo actuators which are complex and expensive in full-size applications.
What is needed is a stabilizer system which is simple, lightweight and inexpensive, which requires little power to operate, and which would not unduly inhibit pilot control for normal maneuvering.